1. Field of the Invention
In general, the present invention relates to on-press developable lithographic printing plates and, more particularly, to on-press lithographic printing plates with microencapsulated developers which allow the plates to be run on a printing press after exposure to actinic radiation without intermediate wet processing steps.
2. Description of the Prior Art
The production of conventional lithographic printing plates, particularly those based upon an aluminum sheet-like support, is well known in the lithographic arts. Such printing plates are typically of the planographic types and printing is accomplished from a substantially flat surface where printing areas are neither raised appreciably above nor depressed appreciably below adjacent and surrounding non-printing areas. In general, these plates comprise hydrophobic (water-repelling) ink-receptive image areas and hydrophilic (ink-repelling) water receptive non-image areas. The hydrophilic non-image areas are typically hydrophilic surfaces bared by a wet (or bath) development process. Thus,non-image areas of photoresist material can, for example, be washed or otherwise removed, to bare a hydrophilic resinous layer, an aluminum (or other metal plate) surface, an anodized aluminum (or other metal plate) surface or a metal plate having a phosphate- or silicate- treated hydrophilic surface.
In the processing of a conventional printing plate prior to use on a printing press, a wet development step will normally be conducted after a photo-exposure step, to remove non-exposed or exposed regions, depending, for example, on whether a negative-working or positive-working photoresist, respectively, is used over the hydrophilic surface. More specifically, the goal of most commercial developing systems for lithographic printing plates is to provide preferential solvation of an organic coating (or resist) which has undergone a photoinduced chemical change. The photoinduced chemical reaction may either reduce or enhance the solubility of the coating, depending on whether the resist is negative-working or positive-working, respectively. In the case of negative-working resists, the solvent must swell and dissolve the unexposed portions of the resist well, but must not swell the exposed portions or distortion of the developed image may result. In the case of positive-working resists, the responses of the unexposed and exposed portions are reversed, but the same principles of preferential solvation apply. The wet development process will usually involve washing and rinsing operations which may be assisted by rubbing or brushing. Other operations such as plate "gumming" may also be performed.
Encumbered by required wet development, the processing of conventional lithographic plates prior to their use on the printing press is time and labor consuming and involves the use of substantial quantities of organic chemicals. It will be appreciated that there is considerable attractiveness for innovations that would satisfactorily eliminate or reduce conventional lithography's long-felt dependency upon the conduct of wet development and thereby permit the use of lithographic plates on a printing press immediately after exposure without required intermediary processing.
Dry lithographic printing plates have been known. These enable the wet processing steps of conventional lithographic printing plates after exposure to be omitted. For example, U.S. Pat. No. 3,793,033, issued to Mukherjee on Feb. 19, 1974, suggests a presensitized, light-sensitive article capable of providing a lithographic printing plate requiring only imagewise exposure to actinic light and no subsequent image development. Mukherjee's plate consists of a support coated with a hydrophilic composition consisting of an organic solvent-soluble phenolic resin and a hydroxyethylcellulose ether, and in reactive association therewith a photoinitiator capable of generating free radicals on exposure to actinic light. Upon imagewise exposure, Mukherjee's coating composition becomes more oleophilic in exposed image areas, while remaining hydrophilic and water-receptive in unexposed areas. U.S. Pat. No. 4,115,127, issued to Ikeda et al. on Sep. 19, 1978, suggests a processing-free lithographic printing plate, which comprises a support having deposited thereon a composition containing germanium and sulfur and at least one of a metal or metal compound in a physically mixed state. As with Mukherjee, exposure to actinic radiation causes relative changes in surface hydrophilicity and oleophilicity.
In contrast to Mukherjee and Ikeda, printing plates based on photosensitive microcapsules have been the subject of prior patents: e.g. U.S. Pat. No. 4,879,201, issued to Hasegawa on Nov. 7, 1989; U.S. Pat. No. 4,916,041, issued to Hasegawa et al. on Apr. 10, 1990; and U.S. Pat. No. 4,999,273, issued to Hasegawa on Mar. 12, 1991. In all cases, oleophilic photopolymerizable compositions were microencapsulated, coated on anodized aluminum substrates, and image-wise exposed through a mask to polymerize and harden the exposed capsules. The photopolymerizable compositions in the non-exposed capsules were then released by either heat or pressure to the hydrophilic aluminum surface and subsequently hardened by a non-imagewise post exposure. A positive image was developed after the imagewise hardened capsules were rubbed off easily by a wet sponge or by the dampening rolls of a lithographic printing press.
While printing plates based on photosensitive microcapsules are dry developable, they are believed to suffer from numerous drawbacks, especially with regard to their durability and resolution. These drawbacks are believed to be accountable to (1) the spreading of the monomer on the high energy aluminum surface, (2) the inhibition of oxygen in the post-exposure step, (3) the high shrinkage and brittleness of the image due to the high monomer concentration in the capsule, and (4) the poor wet adhesion due to the high concentration of water soluble polymers as binders for the capsule coating.